Project Summary Regulatory T (Treg) cells characterized by the expression of the transcription factor forkhead box P3 (Foxp3) play a central role in the control of immune tolerance to self-antigens, allergens, and commensals as well as immune responses to pathogens and tumors. mTORC1 is a master growth regulator that is essential for Treg cell function in vivo, as mice with Treg cell-specific loss of mTORC1 develop rampant autoimmune disease and die at an early age. mTORC1 integrates signals that reflect the availability of growth factors and the nutritional status of an organism. Amino acids is an essential nutrient metabolite that makes up the majority of mammalian cell mass and functions as critical regulator of mTORC1 signaling. Nevertheless, whether amino acid sensing plays a role in Treg cell function remains elusive. To this end, this proposal will study the role of the Rag GTPases dependent amino acid-sensing pathway in Treg cell homeostasis and function. Preliminary studies revealed that constitutive activation of this pathway resulted in deregulated Treg cell homeostasis and precipitated immune activations in mice, most prominently at sites where Treg cells are constantly bathed in fluctuating levels of nutrients, such as the liver and the intestine. It is thus hypothesized that repression of the amino acid-sensing pathway is essential for Treg cells to maintain stable expression of Treg cell characteristic molecules and suppressive function, and the lack of this negative regulation results in overt immune activation. This proposal seeks to characterize how constitutive activation of the amino acid-sensing pathway affect Treg cells by characterizing the proliferation, survival, and suppressive function of Treg cells. Furthermore, this study will investigate if deregulation of the amino acid-sensing pathway bypasses the stringent requirement of strong T cell receptor and IL-2 signaling in proliferating Treg cells, resulting in the generation of defective Treg cells. Finally, this study will investigate if constitutively activated amino acid sensing affects Treg cell functions in chronic viral infection and cancer. Together, this research will shed light on how the sensing of amino acid affects Treg cell homeostasis and function. From a clinical standpoint, altered Treg cell homeostasis can contribute to the pathogenesis of a variety of human diseases including infections and cancers. Thus, this proposed research may uncover ways to modulate immunity and inflammation for therapeutic benefit.